Cellulosic materials (natural and synthetic) in different forms (fiber, film, powder, particle, pellet, chip, etc) at different sizes (nano, micro or macro) are often flammable and have low thermal resistance. They can be burned and also can spread the fire in the presence of oxygen. Thus, their use either in direct or non-direct form is limited in applications requiring fire resistance. Due to their flammability, the use of cellulosic materials in polymer composites is also limited in certain applications.
Cellulosic materials are treated with different flame retardants depending on the application, for example in furniture, textiles or composites. The most commonly used flame retardants are based on halogen (e.g. WO 2004/1097088), phosphorous (e.g. U.S. Pat. Nos. 3,899,483; 6,524,653; 4,228,202; 5,238,464; US 2005/0215152; US 2004/0094279; WO 2002/1044470; U.S. Pat. No. 6,352,786), boron (e.g. WO 2005/1042840; U.S. Pat. No. 4,228,202; US 2009/0156073), ammonium (e.g. U.S. Pat. Nos. 4,228,202; 6,524,653; WO 2002/1044470), graphite (e.g. JP 09-031887; EP 0735187), alkaline-earth metallic compounds (e.g. U.S. Pat. No. 5,434,200) or mixtures thereof. To improve fire resistance of organic polymer composites, the incorporation of flame retardants based on halogen, phosphorous, metallic hydroxide (magnesium hydroxide, aluminum hydroxide, calcium hydroxide, layer double hydroxide), metallic oxide (antimony oxide, boron oxide), silicate (clay, talc), etc, in the polymer matrix has been widely used.
Among the compounds listed above, halogen based flame retardants are well known to be the most efficient as they can be used at a low concentration in the final composition thus limiting their impact on other properties of the product. However, halogen compounds are considered to be harmful to the environment. Boron compounds are supposed to be efficient, however they tend to be washed off due to their good solubility in water. Less harmful flame retardants based on phosphorous, graphite or alkaline-earth metallic compounds are much less efficient, thus a large amount of those additives must be used in the formulation. The use of flame retardant incorporated in a polymer matrix alone does not satisfactorily resolve the flammability problem in cellulose-polymer composites, especially when the concentration of cellulose is quite significant in the formulation of the composite.
It is generally known that metal hydroxides, including barium hydroxide, can be used as a flame retardant for cellulosic materials (e.g. US 2009-298370; U.S. Pat. No. 671,548; Chen 1991; Mostashari 2008) and for polymer materials (e.g. U.S. Pat. No. 7,354,958). Further, CA 2,079,302 discloses a flame retardant composition for cellulosic material comprising sodium hydroxide and a metal salt of boron among other ingredients. The metal salt of boron is defines as borax which is a sodium tetraborate. U.S. Pat. No. 3,973,074 discloses a flame-proofing composition comprising potassium hydroxide and/or potassium carbonate and possible a small amount of sodium hydroxide and/or sodium carbonate and may include another potassium salt. U.S. Pat. No. 5,480,587 discloses inorganic additives to impart flame resistance to polymers. The additives include hydroxides and metal salts that evolve gas. One such metal salt is barium chloride dihydrate. U.S. Pat. No. 4,567,242 discloses the use of a mixture of a polycondensate of a halogenated phenol and an alkaline earth metal halide in a flame retarding composition.
US 2003-0220515 discloses flame retardant compositions in which ancillary flame retardant additives may be used alone or in combination, such as metal hydroxides and metal salts, including alkaline earth metal salts. There is no disclosure of the use of an aqueous mixture of alkali metal hydroxides with alkaline earth metal salts.
U.S. Pat. No. 4,064,317 discloses the use of “alkali compounds” for use in flame resistant plaster board. The “alkali compounds” are defined as at least one of an alkali metal hydroxide, alkali metal salt, alkaline earth metal hydroxide or alkaline earth metal salt. It is preferred to use a mixture of alkali metal salts and alkaline earth metal salts, for example a mixture of sodium and calcium formate. The combination of alkali metal hydroxide and alkaline earth metal salt, especially barium salts, is not specifically disclosed.
The abstract of CN 1869154 discloses a flame retardant composition which initially involves the step of making magnesium hydroxide from the reaction of magnesium sulfate and sodium hydroxide. However, this document does not disclose treating a cellulosic material with an aqueous reaction mixture of an alkali metal hydroxide and alkaline earth metal salt simultaneously with or shortly after mixing the alkali metal hydroxide with alkaline earth metal salt.
It is known that treatment of cellulosic materials with alkaline earth metal carbonates (e.g. barium carbonate) imparts fire resistance to the cellulosic material (e.g. Mostashari 2004-2005). Here, the alkaline earth metal carbonate is applied to the cellulosic material by first coating the cellulosic material with an alkaline earth metal chloride and then treating the so-coated material with sodium carbonate. It is also known to use both a clay and a metal hydroxide in a fire retarding composition comprising a polymer material (e.g. GB 2367064; JP 2002-180374).
There remains a need for an environmentally friendlier, effective approach to producing fire-resistant cellulosic materials.